This invention relates to a novel process for the preparation of tricalcium phosphate (hereinafter referred to as TCP). Commercially available TCP has been unsatisfactory in the past, due to the tendency to agglomerate upon drying, and, when incorporated in powdered mixes, such as dry beverage mixes, to dissolve slowly and incompletely in the presence of food acids, such as citric acid, and produce turbidity and sediment in reconstituted beverages.
Calcium phosphates (mono, di, tri and hydroxyapatite) are widely used in everyday life. The fairly water insoluble TCP and hydroxyapatite are the major mineral constituents of bone and teeth in the Animal Kingdom. The solubility of calcium phosphates in water, decreases with the increase of the degree of neutralization of the phosphoric acid (e.g., monocalcium phosphate is the most soluble in water; hydroxyapatite is the least soluble in water). All the calcium phosphates are soluble in strong mineral acids, such as hydrochloric acid. However, when relatively weak food acids, such as citric acid, are used, the rate and degree of solublization of the calcium phosphates is greatly dependent upon their degree of neutralization. The very basic phosphate, calcium hydroxyapatite, is extremely slow to dissolve in citric acid, and a drink without sediment or turpidity is impossible to maintain.
The commercial process presently used for the production of TCP is well known, and involves the slow addition of phosphoric acid to a lime slurry at a temperature between 70.degree. C. and 80.degree. C. until the pH is nearly neutral, then filtering and drying the resultant wet cake, which has an overall solids composition corresponding to a CaO:P.sub.2 O.sub.5 mole ratio of 3. The dried product is then milled in one of several ways to reduce the average particle size and render the TCP suitable for use.
It is believed that the incomplete solubilization of commercial TCP in acidic food solutions is due to the presence of the very insoluble, crystalline, basic tricalcium phosphate, also known as calcium hydroxyapatite (hereinafter referred to as hydroxyapatite).
It is generally recognized that crystals are formed in two stages: a nucleation stage (formation of the incipient crystal) and a 3-dimensional growth stage. The nucleation stage is believed to require a certain degree of supersaturation to overcome an "activation" energy in order to form a nucleus (a minimum grouping of ions that is capable of subsequent growth into a crystal). Following nucleation, crystal growth then proceeds as long as the solution is supersaturated with respect to that particular crystal composition. Crystal growth is a very complicated process that is governed by many factors. However, at a given set of conditions, the rate of crystallization is greatly accelerated as the number of nuclei present in solution is increased. Also, an increase in temperature results in an increase in the rate of crystal proliferation.
In U.S. Pat. No. 3,387,925, to Vanstrom et al., a method was disclosed which produced a TCP with a smaller particle size by controlling the reaction temperature and controlling the final pH. The process described therein favored the precipitation of a fine particle size TCP and obviated the need for drying and milling.
U.S. Pat. No. 4,324,772 to Conn et al., discloses a process for producing tricalcium phosphate/hydroxyapatite in large volumes. The process comprises continuously charging to the first stage of a two stage reactor, a slurry of calcium oxide in water and a solution of phosphoric acid in water. The lime slurry and phosphoric acid in water are allowed to react in the first stage under vigorous agitation at a pH such that the viscosity of the reaction mass is near minimum viscosity. The reaction is continued in the second stage of the reactor under vigorous agitation.